A R C H I V E S

o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 10

Issue 2/2010

141 – 146

24/2

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 2 / 2 0 1 0 , 1 4 1 - 1 4 6 141

Application of heat treatment and hot

extrusion processes to improve mechanical

properties of the AZ91 alloy

T. Reguła

a,*, E. Czekaj

a, A. Fajkiel

a, K. Saja, M. Lech-Grega

b, M. Bronicki

c

aFoundry Research Institute, Kraków, Poland,

bInstitute of Non-Ferrous Metals-Light Metals Division, Skawina, Poland,

cAGH University of Science and Technology, Kraków, Poland

*Corresponding author. E-mail address: tregula@iod.krakow.pl

Received 22.04.2010; accepted in revised form 10.05.2010

Abstract

The main aim of this paper is to evaluate the effects of hot working (extrusion) and hest treatment on room temperature mechanical

properties of magnesium-based AZ91 alloy. The results were compared with as-cast condition. The examined material had been obtained

by gravity casting to permanent moulds and subsequently subjected to heat treatment and/or processed by extrusion at 648 K.

Microstructural and mechanical properties of properly prepared specimens were studied. Rm, Rp02 and A5 were determined from tensile

tests. Brinell hardness tests were also conducted. The research has shown that hot working of AZ91 alloy provides high mechanical

properties unattainable by cast material subjected to heat-treatment. The investigated alloy subjected to hot working and subsequently

heat-treated has doubled its strength and considerably improved the elongation - compared with the as-cast material.

Keywords: Magnesium Alloy, Heat Treatment, Hot Working

1. Introduction

Over the past few years (2000÷2006), the world production of

magnesium has been reported to enjoy an average annual growth

of about 14% [1]. Because of low mechanical properties, pure

magnesium has not found any wider application as a structural

material. On the other hand, in alloyed form, it is used for casting

and plastic working. The most beneficial feature of magnesium

alloys is their extremely low density of about 1,8 g/cm3 (it is – as

a matter of fact – the lowest density among all the commercial

alloys) [2]. It is combined with a supreme specific strength, good

machinability and themal conductivity, easy recycling, good

damping capacity and ability to absorb electromagnetic waves [3].

These are the reasons why magnesium alloys are becoming the

preferred engineering material in automotive industry, where the

reduced weight of elements means less of fuel consumption, and

hence lower rate of the greenhouse gas emissions.

The majority of intricate parts made from magnesium alloys are

fabricated by various casting processes, like gravity casting into

metal and sand moulds, pressure die casting, squeeze casting, or

semi-solid (thixocasting) process [4]. The reason that lies behind

this fact is rather poor plastic deformability at room temperature

of magnesium and its alloys, which considerably limits the

applicability of cold working processes.

Magnesium crystallizes in the hexagonal system; the ratio of

an elementary cell parameters c/a is 1,624, which means that

packing of atoms in the lattice is close to an ideal condition. The

deformation at room temperature is very limited; almost only

along the planes of the hexagon base (0001)<1120>. In this

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situation, only three slip systems are available, which is not

sufficient for a metal to be considered plastic since it should have

at least five independent slip systems [5]. This is the reason why

magnesium alloys are suitable for plastic working only at high

temperatures, when the slip along the side (prismatic) and internal

(pyramidal) planes becomes possible, making them finally plastic

[6].

During high-temperature metal deformation, several phenomena

occur at the same time: hardening, dynamic recovery (DRV) and

dynamic recrystallisation (DRX). Magnesium alloys are

characterised by the low stacking fault energy (60÷78 kJ/mol),

and therefore it is the dynamic recrystallisation that plays the

leading role in their hot plastic working (above 513 K) [7]. DRX

is responsible for an abatement of the deformation effects; it

improves ductility and reduces the resistance to flow, thus

enabling the deformation process to proceed without the need for

continuous increasing of external forces [3].

Because of high aluminium content (~ 9,0 wt.%), AZ91 is

considered typical cast alloy [8]. The research described in this

paper aims not only at the determination of its mechanical

properties after heat treatment or extrusion, and comparing them

with as-cast condition, but also at proving that the division into

cast alloys and alloys for plastic working is the matter of purely

conventional agreement.

2. Methods

The investigations were made on AZ91 magnesium alloy. Its

chemical composition is given in Table 1.

Table 1.

Chemical composition of the examined alloy [9]

The cylindrical specimens of Ø 60 mm dia. were cast in

permanent moulds and machined to a final diameter of Ø 40 mm.

The specimens of the required dimensions were subjected to the

process of hot direct extrusion carried out at the Department of

Non-Ferrous Metals, University of Science and Technology in

Cracow.

The following process parameters were observed: elongation

factor - 16, temperature – 648 K, and ram feed rate – 0,5 mm/s.

Thus produced wire of Ø 10 mm dia. was cut into 110 mm x 10

mm specimens for mechanical tests (reference line – 50 mm), and

into the specimens for hardness measurements and

microstructural examinations. Half of the specimens were

subjected to a heat treatment, i.e. ageing at 343 K for 16 hours

under the argon protective atmosphere (condition: T5).

The values of the mechanical properties of AZ91 alloy in the

starting condition and after heat treatment (conditions: T4, T5,

T6) were taken from earlier studies on this subject [9, 10]. The

heat treatment was conducted according to Standard Practice for

Heat Treatment of Magnesium Alloys [11]. The solutioning to

condition T4 was carried out at a temperature of 689 K for 16 h;

the parameters of alloy ageing to condition T5 were as follows:

temperature – 343 K, time – 16 h. The material after solutioning

was aged to the precipitation hardened state (T4 to T6) at

a temperature of 343 K for 16 h. The heat treatment process was

carried out under the argon protective atmosphere.

Static tensile tests at room temperature were conducted on an

INSTRON 1115 machine according to PN-EN 10002-1:2004 at

a rate of 0,6 mm/min. Four specimens were tested in each test

variant. Hardness was measured by Brinell method using a 2,5

mm diameter indenter and a load of 625 N according to PN-EN

ISO 6506-1:2002. In each test variant six measurements were

taken on the specimen cross-sections.

Microstructure was examined under an OLYMPUS DP70

optical microscope at the Institute of Light Metals in Skawina.

The examinations were preceded by standard grinding and

polishing of specimens, which were next etched in nital (3%

solution of nitric acid in ethyl alcohol). Percent fraction of the

Mg17Al12 phase precipitates was calculated by a grid method

according to PN-84/H-04507/01.

3. The results and discussion

microstructural examinations

Representative microstructures present in the three variants of

AZ91 alloy are shown in Figure 1 in function of the processing

technique. The structure typical of the starting (as-cast) condition,

i.e. the 0 structure, is shown in Figure 1a. Its composition includes

the solid solution of αMg (light colour) and large, dark-coloured,

precipitates of an intermetallic equilibrium Mg17Al12 phase –

present mainly on grain boundaries.

The microstructures of alloy subjected to plastic working, i.e.

to hot extrusion (PP), are shown in Figures 1b and 1c. The

structure of PP specimens differs quite considerably respective of

the starting condition. At the same magnification, the results of

very severe plastic deformation suffered by the examined alloy

are very well visible. The precipitates of Mg17Al12 phase in

specimens subjected to plastic working are finer and characterised

by much higher degree of dispersion. The microphotograph in

Figure 1b does not allow the grain size to be exactly determined,

but knowing that the Mg17Al12 phase is mainly present on grain

boundaries one can assume that the size of the grains has

decreased quite considerably.

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Figures 1d and 1e show microstructures observed in

specimens of AZ91 alloy extruded and then subjected to heat

treatment (PPS) at a temperature of 343 K for 16 h. Compared

with microstructures of alloy hot worked but without heat

treatment, the morphology of Mg17Al12 phase precipitates has

changed. The percent fraction of this phase in the structure has

increased from 22,7% (PP) to 47,6% (PPS) with the precipitates

undergoing partial coagulation. This indicates a significant degree

of supersaturation of the αMg solid solution with an alloying

element during alloy cooling after extrusion. The images of

microstructures observed in PP and PPS specimens on their

longitudinal sections (Figs. 1c and 1e) show, typical of the

extruded material, considerable grain elongation in the direction

of extrusion, caused by severe plastic deformation to which the

examined alloy has been subjected.

a)

b)

c) d)

e)

Fig. 1. Microstructures of AZ91 alloy; a) as-cast condition, mag. 200x; b) hot-worked, cross-section, mag. 200x; c) hot-worked,

longitudinal section, mag. 50x; d) hot-worked and heat-treated, cross-section, mag. 200x; e) hot- worked and heat- treated, longitudinal

section, mag. 50x

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4. Testing of mechanical properties

The results of the tests are given in Table 2, while Figure 2

depicts them in a graphic form. Table 3 shows percent changes in

the properties of the examined alloy respective of its as-cast

condition and in function of the processing treatment type.

From Figure 2 it follows that the examined alloy in as-cast

condition can offer rather poor mechanical and plastic properties.

This is directly related with its coarse-grain structure described

above. Its tensile strength at a level of 167 MPa and the yield

point Rp0,2 reaching 81 MPa are indeed the values much too low to

make AZ91 useful in as-cast condition for structural applications,

especially if taking into consideration the fact that the very

popular and cost effective alloys from an Al-Si system are in most

cases capable of offering much better properties.

Table 2.

Mean values of the mechanical properties of AZ91 alloy and their standard deviations [9]

Fig. 2. Graphic representation of relationships between mechanical properties and conditions of AZ91 alloy [9]

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A maximum heat-treated strength offers the alloy after

solutioning and artificial ageing to condition T6. Increasing

further the strength of gravity cast AZ91 alloy through heat

treatment is very difficult. Having Rm at a level of up to 300 MPa

is feasible only through properly applied plastic (thermo-plastic)

working.

The examined alloy extruded at a temperature of 648 K (PP)

is characterised by very good mechanical properties and excellent

ductility. Its ultimate tensile strength is 312 MPa and the yield

point Rp0,2 – 183 MPa, which means percent increase of 87% and

126%, respectively, compared with the as-cast (starting)

condition. The elongation on 5-fold specimens has reached the

value of 13%, which means a gain of 225%. Taking into account

the low density of AZ91 alloy, amounting to about 1,81 g/cm3,

this result is very satisfactory. No doubt that this considerable

improvement in properties is the result of a combined effect of

different factors. First, it means eliminating through hot working

the casting defects, adversely affecting the mechanical and plastic

properties of the alloy. Next, it means strong grain refinement,

which affects the alloy hardening behaviour, especially in

hexagonal system [12]. This is due to the fact that in A3 system,

at room temperature, the slip is practically possible along one

single plane only (0001), which considerably complicates the

propagation of deformation (slip engagement) in other grains.

The observed hardening of material is also due to a change in

the morphology of Mg17Al12 phase precipitates, i.e. diminishing

of their size and strong dispersion. The next factor contributing to

alloy hardening during hot working is the visible elongation of

grains, which follows the extrusion process and causes some

anisotropy of alloy properties. The more elongated are the grains,

the higher are the mechanical properties of the material in

direction of the extrusion [13]. However, the elongation of grains

brings some adverse effects, too, e.g. strength variations (SDE -

Strength Differential Effect) [14]. The material affected by SDE is

characterised by lower yield point in compression compared with

tension.

Further improvement in mechanical properties of the

examined alloy was obtained when the extruded specimens were

subjected to a heat treatment comprising artificial ageing. Owing

to this treatment, the yield point increased by 13% (compared

with PP specimen) and Rm by 3%, ufortunately on the cost of

elongation, which dropped slightly. The reason was increased

number of the Mg17Al12, phase precipitates, which coagulated and

formed clusters, making both ultimate tensile strength and

ductility drop.

Table 3.

Percentage changes in mechanical properties of AZ91 alloy in function of the processing type, referred to as- cast condition

5. Conclusions

The results of the investigations described in this study enable

the following conclusions to be drawn:

it is possible to subject the cast AZ91 alloy to plastic

working by hot extrusion,

the application of hot plastic working (extrusion) enables

obtaining the mechanical properties (plastic properties, in

particular) unattainable for products made from AZ91

alloy when in as-cast condition,

the heat treatment of AZ91 alloy subjected to plastic

working gives but only very modest results (especially as

regards the increase of yield point).

Acknowledgements

The authors are greatly indebted to Professor Dr Sc. Eng. Henryk

Dybiec from the University of Science and Technology for his

most valuable assistance in studies of the plastic working of AZ91

alloy.

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